This invention relates generally to gas turbine engines, and more particularly to internally cooled airfoils used in gas turbine engines.
Gas turbine engines, such as aircraft jet engines, include many components (e.g., turbines, compressors, fans and the like) that utilize airfoils. Turbine airfoils, such as turbine blades and nozzle vanes are exposed to the highest operating temperatures and thus typically employ internal cooling to keep the airfoil temperatures within specified design limits. The design limits define an acceptable balance between factors such as operating efficiency, wear longevity and heat tolerance.
A turbine rotor blade, for example, has a shank portion that is attached to a rotating turbine rotor disk and an airfoil blade portion that extracts energy from the hot gases exiting the engine's combustor. The airfoil includes a blade root that is attached to the shank and a blade tip that defines the free end of the blade. The airfoil of the turbine rotor blade is cooled by air, typically bled from the engine's compressor, that passes through an internal circuit in the airfoil. The air enters near the airfoil blade root and exits near the airfoil blade tip, as well as through film cooling holes near the airfoil blade's leading edge and through trailing edge cooling holes. Known turbine blade cooling circuits include a plurality of radially-oriented passageways that are series-connected to produce a serpentine flow path, thereby increasing cooling effectiveness by extending the length of the coolant flow path.
It is also known to provide additional, unconnected passageways adjacent to the serpentine cooling circuit. Turbine rotor blades with internal cooling circuits are typically manufactured using an investment casting process commonly referred to as the “lost wax” process. This process comprises enveloping a ceramic core defining the internal cooling circuit in wax shaped to the desired configuration of the turbine blade. The wax assembly is then repeatedly dipped into a liquid ceramic solution, causing a hard ceramic shell to incrementally form on the surface of the assembly. When the proper thickness is achieved, the wax is melted out of the shell so that the remaining mold consists of the internal ceramic core, the external ceramic shell and the intermediate empty space previously filled with wax. The empty space is then filled with molten metal. After the metal cools and solidifies, the external shell is broken and removed, exposing the metal that has taken the shape of the void created by the removal of the wax. The internal ceramic core is dissolved via a leaching process. The metal component now has the desired shape of the turbine blade and the formed internal cooling circuit.
In casting turbine blades with serpentine cooling circuits, the internal ceramic core is formed as a serpentine element having a number of long, thin branches. This presents the challenge of making the core sturdy enough to survive the pouring of the metal while maintaining the stringent requirements for positioning the core. Moreover, some prior art airfoils include three cooling circuits, a leading edge circuit, a mid chord circuit and a trailing edge circuit. The mid chord circuit is an axial serpentine circuit flowing from the trailing edge to the leading edge, and the leading edge and trailing edge circuits are impingement-type circuits.
In this design, the pressure sidewall and the suction sidewall of the mid chord circuit contact the same cooling air at the same temperature. This design is relatively easy to cast, but does not provide sufficient flexibility to control the temperatures of the pressure sidewall and the suction sidewall effectively and efficiently.
Other prior art designs provide separate flow control on the pressure side and the suction side. These designs require an assembly core within the mid chord circuit. The assembled mid chord circuit is then assembled with other circuits to for a complete casting core.
Accordingly, there is a need for a turbine airfoil in which different cooling circuit combinations can be formed in one ceramic core, that provides more control of local temperatures on various parts of the airfoil, and that can be cast using conventional processes.